The word "愣头愣脑" is formed by combining two parallel phrases: "愣头" (rash head) and "愣脑" (rash brain), where the adjective "愣" (reckless) is repeated and paired with synonymous nouns "头" and "脑" (both referring to the head or mind), creating a reduplicative structure that intensifies the meaning to describe someone as impulsive or foolishly bold.